Here's a step by step guide on how one goes about bleeding atv brakes. There are actually only 3 components you'll be working with. 2 bleed valves (1 on each brake caliper) with rubber caps on, and 1 brake reservoir.
The hole purpose of bleeding brakes is to get rid of air inside the brake lines. So, you'll have to do everything in your power to prevent air from going in while you'r working on bleeding your atv brakes. This is crucial in step 4. Just keep that in mind.
1. Make sure all the connections of the brake lines are tight, without leaks. Make sure everything is installed properly.
2. REMOVE RUBBER CAPS/NIPPLES:
The bleed valves, covered by rubber caps, are located on the top of each brake caliper. Remove the rubber cap on each valve.
3. FILL REVERVOIR:
Remove the reservoir cap from the brake reservoir (located on the handlebars on most atv's) and fill it to the top with DOT 4 brake fluid. Make sure the reservoir always contains brake fluid to prevent air from entering the brake lines. So keep an eye on that during the process. Refill the reservoir in time.
4. TAP RESERVOIR:
Gently tap the brake reservoir and shake the brake lever up, down, left and right. You should see air bubbles coming up from the master cylinder where the fluid enters the line. This means fluid is traveling into the lines and down to the calipers, while air is surfacing. Do this for some time and make sure to keep the master cylinder filled. Refill it in time, to prevent air from entering.
5. PUMP THE BRAKES:
Once most of the air is out, pump the brakes. Pump the brakes with extremely slow pulls. Make each pump take 2 seconds, from the lever all the way extended to the grip. If you do it fast, not as many air bubbles are going to get out of the system. If you open the reservoir cap, you can see the bubbles coming out of the line and into the brake reservoir. Squeeze and release the brake lever 15 - 20 times. On the final squeeze hold it in!
6. HOLD LEVER - BLEED VALVE - RELEASE LEVER:
While holding in the lever, open one of the bleed valves on a caliper with an 8mm wrench. You should hear air & see fluid ease out. Once the air stops easing out, tighten the valve, ...and release the brake lever.
7. Repeat steps 4, 5 and 6 for the other caliper. Keep at it until you get a good amount of braking action in the lever, and untill all the air is out of the lines.
Your first try bleeding atv brakes will probably take some time. But once you get the hang of it you won't ever forget the technique to it.
Also, when you're done. Go for a ride. The vibrations when riding, will send some more unnoticed bubbles up. So bleed once again and you'll end up with a safe and perfect braking brakesystem!